The invention relates to methods of fabricating highly uniform, small metallic balls, and to the balls themselves, from capillary stream break-up at high rates.
The generation of droplets from capillary stream break-up has been studied at least as early as Lord Rayleigh in the 1800s. More recently, the formation of metallic spheres, or balls, from the break-up of a molten metal capillary stream has been studied. Such balls are commonly used in the electronics industry for various applications, including interconnects for small electronics packages and in the manufacture of conductive pastes. Using the process of capillary stream break-up, the balls can be produced at very high ratesxe2x80x94typically tens of thousands of droplets per second. Further, the nature of droplet formation due to capillary stream break-up results in highly uniform balls. The highly uniform size of the metal balls formed from capillary stream break-up is a significant improvement over other methods of forming conductive powdersxe2x80x94such as spray atomization or melt spinningxe2x80x94which require the extra step of sieving or sorting the differently sized balls. This extra step is labor intensive, significantly increasing the time and cost of the manufacturing process; however, with such technologies, sorting or sieving is necessary to achieve tight ball diameter tolerances (on the order of five percent).
In the production of metal balls from capillary stream break-up, it is advantageous to effectively cool the balls so that they solidify before landing or bonding with each other. Effective solidification reduces or eliminates (1) irregularly shaped balls that have dented when they impinge and (2) irregularly sized balls that have bonded together because they were insufficiently cooled. Without effective solidification, removal of these defects requires that the balls be sieved or sorted.
A method for producing highly uniform metallic micro-spheres, or balls, comprises directing a capillary stream of molten metal from an orifice, wherein a plurality of droplets form from the stream due to capillary stream break-up; electrostatically charging the plurality of droplets; and deflecting the plurality of droplets by directing the droplets through an electric field. In one innovative aspect of the present invention, each droplet travels along a path that is different than the path of the adjacent downstream droplet; and each droplet is captured after the droplet has solidified. In another innovative aspect, the charge applied to each droplet is varied while the electrostatic field remains constant over time. In yet another innovative aspect, the charge applied to the droplets is constant while the electrostatic field is varied over time.
In a preferred embodiment, the electric field is created by a pair of deflection plates, across which a bias voltage is applied. In accordance with another embodiment, the electric field is created by two orthogonal pairs of deflection plates, across each of which a bias voltage is applied. In yet another embodiment, the electric field is created by a pair of rotating deflection plates. In a further embodiment, the droplets may be directed to pass through a chamber filled with a cooled gas.
In a further innovative aspect of the present invention, the metal balls are produced at a rapid rate, wherein the balls are highly uniform, having highly uniform diameters. More particularly, the balls are produced, depending on orifice size and forcing conditions, at a rate preferably in a range of about 1000 to 200,000 balls per second, and preferably at a rate greater than about 4000 balls per second, while maintaining a ball diameter tolerance preferably in a range of about 0.5 to 3.0 percent, and preferably less than about 2.0 percent, without performing a mechanical sieving or sorting step.
Other aspects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.